The OnePlus 7T family received their third OxygenOS Open Beta update last week, which introduced a handy camera lens dirt detection feature as well as the April 2020 security patches. However, as it turns out, the company secretly enabled the 960fps slow motion video recording support on the regular OnePlus 7T with this update, albeit the feature is not at all mentioned in the OTA changelog.
A Redditor with the screenname u/sneakerspark first noticed that with Open Beta 3, they now have a 720p@960fps slow motion option on the stock camera app. The person also confirmed in a follow-up reply that they have 4K@30fps wide-angle video. Neither of these changes made it to the official changelog, which is quite surprising. For your information, OnePlus did tell us back in September 2019 that they were working on these two features. Perhaps the current implementation needs another coat of paint before OnePlus is confident enough to announce them officially.
Note that the camera sensors on the OnePlus 7T, including the primary 48MP Sony IMX586 module, don't carry enough horsepower to actually record 960 frames each second. They lack the dedicated DRAM die needed to temporarily store the massive number of frames that are being captured before passing them to the image buffer and finally writing them to storage. Fortunately, they are capable of handling 720p@480fps, thus OnePlus only needed to incorporate some kind of frame interpolation algorithm. The final result might not as crisp as native 960fps slow-motion, but it shouldn't matter to most people.
Regarding wide-angle video recording, the original OnePlus 7 Pro also lacked the feature initially, but the company later enabled the functionality through the Android 10 update for the device. It is good to see that OnePlus is trying to bring over feature parity across the different rear cameras, even on their older phones.
Source: r/Android